Question

How did early chemists determine which substances were elements?
by observing chemical reactions,
by finding the atomic mass,
by counting neutrons, or
by counting protons

Answer 1

Finding the atomic mass would do you no good at all, and you can't count neutrons and protons without an amazing microscope that hasn't yet been invented. So you're stuck with observing chemical reactions.

However, no early chemist would have said he could "determine whcih substances were elements." The best you can say is that you think a certain substance is an element, because you are unable to reduce it to two or more other substances by some chemical reaction. So you might propose a substance as an element -- subject to future tests.

And indeed, the early history of chemistry is full of cases where substances were misidentified as "elements" because no one had yet figured out a way to separate them into other substances. Air was regarded as an element for a long time, because no one properly understood combustion. (It was generally considered a process in which solid substances released something into the air, rather than something in the air combining with the solid.) For a long time, the oxides of metals, called "earths," were considered the "elements," and the metals themselves as being combinations of these "earths" and other elements, like fire, water or air.

Then there is the case of the noble gases, oly discovered near the turn of the 20th century, unsuspected because all of them are extremely unreactive -- do not participate in any chemical reactions at all. They were only discovered when extremely precise measurements of liquified air revealed that there was a tiny residuum that could not be reacted with anything.

In short, observing chemical reactions gets you closer and closer to identifying elements, but it's always a process of elimination -- of paring down the substances you can call "elements" until you are, hopefully, left with those that really are. With the advent of more direct methods of probing the atom these days, that is no longer an issue.

Answer 2

by finding the atomic mass,

Answer 3

thnx

Answer 4

the correct answer is, by observing chemical reactions